1. Field of the Invention
The present invention relates to a gain control amplifier.
2. Description of the Background Art
FIG. 13 is a circuit diagram of a conventional gain control amplifier. An input signal V.sub.in applied to a signal input terminal 1 is transmitted to an inverting input of an amplifier 3 through an input resistor 2. A reference power supply 6 is connected to a non-inverting input of the amplifier 3, and a feedback resistor portion 4 is connected between the inverting input and the output of the amplifier 3. An output signal V.sub.out is given to a signal output terminal 5. The relation between the input signal V.sub.in and the output signal V.sub.out is as follows: ##EQU1## where Z is a resistance of the feedback resistor portion 4.
When switches K.sub.5 to K.sub.8 are off, the resistance Z is equal to a resistance R and Equation (1) is expressed as: ##EQU2## When the switches K.sub.s to Ks are on, the resistance Z is equal to a value provided by parallel connection of five resisters of resistances R, 2R, 4R, 8R, 16R, respectively, and Equation (1) is expressed as: ##EQU3## Assuming that r.sub.0 =R, a controllable gain G range is given from Equations (2) and (3) as: ##EQU4## It is thus seen that the gain is linearly controllable in the range of 1 to about 1/2 times.
In the above conventional gain control amplifier, the feedback resistor portion 4 requires a resistance sixteen (2.sup.4) times greater than the input resistance R where r.sub.0 =R. In FIG. 13, the gain is shown as controlled in 15 (=2.sup.4 -1) steps. For finer control, for example (2.sup.n -1) step control, the feedback resistor portion 4 requires a resistance 2.sup.n times greater than the input resistance R.
The conventional gain control amplifier is disadvantageous in that finer, linear control of the gain necessitates a resistor of an extremely great resistance.